FIG. 7 is a schematic diagram that depicts a general configuration of a satellite communication system. A feeder link station 4 is a base station for controlling a satellite repeater 5, and each of terminal stations 6 and 7 present in service areas 1 to 3. In this case, it is assumed that the satellite communication system is a single-hop system, although in a case of a double-hop system where communication is established between terminals via a feeder link, the feeder link station 4 sends and receives a communication through a communication channel from each of the terminals.
A plurality of terminal stations (for example, a small satellite station, a mobile phone terminal, an aircraft, a bullet train, or a ship) that use a service of the satellite communications are in the service areas 1 to 3. It is conceivable a case where one service area includes a plurality of terminals. For simplifying explanations, in this case, it is assumed that the satellite communication system includes three service areas, although a system that includes tens of service areas is conceivable because of recent achievement of a multi-beam system.
The feeder link station 4 establishes communications between service areas via a satellite, or communications between a service area and the feeder link station by controlling connections between terminals via the satellite repeater 5 and controlling a transmission and a receipt between terminals present in the service areas 1 to 3.
According to FIG. 7, communication channels 11 to 13 and 71 to 73 between the service areas 1 to 3 and the satellite repeater 5 are referred to as service links, and communication channels 14 and 74 between the feeder link station 4 and the satellite repeater 5 are referred to as feeder links. The service links 11 to 13 and the feeder link 14 from the terminals in the service areas 1 to 3 and the feeder link station 4 toward the satellite repeater 5 constitute an up-link beam, and links from the satellite repeater 5 toward the terminals in the service areas 1 to 3 and the feeder link station 4, namely, the service links 71 to 73 and the feeder link 74, constitute a down-link beam.
FIG. 8 is a schematic diagram that depicts a sequence in a case where the terminal station 6 in the service area 1 carries out a communication with the terminal station 7 present in the service area 3 as a concrete example of a communication sequence of the system.
The terminal station 6 in the service area 1 transmits a transmission request signal to the feeder link station 4 via the satellite repeater 5 (Step S1). The transmission request signal includes information about a bandwidth desired to be used, a transmission destination (the terminal station 7), and a transmission source (the terminal station 6), as well as transmission request information.
The feeder link station 4 has grasped all terminals connected to the system and a state of the use of frequencies in each service area, and when receiving the transmission request signal from the terminal station 6, the feeder link station 4 examines at first whether the terminal station 7 as the transmission destination exists in the system (Step S2).
If the terminal station 7 exists in the system (Yes at Step S2), the feeder link station 4 examines whether allocation of a frequency band in accordance with the request from the terminal station 6 is available on both an up-link (from the terminal station 6 to the satellite repeater 5), and an down-link (from the satellite repeater 5 to the terminal station 7) (Steps S3 and S4: examining an available frequency). If the terminal station 7 does not exists in the system (No at Step S2), the processing is terminated.
When the allocation of a frequency band is available on both the up-link and the down-link (Yes at Step S4), the feeder link station 4 then transmits a transmission request signal to the terminal station 7 in the service area 3 via the satellite repeater 5 (Step S5). The transmission request signal includes information about a frequency band to be used, a transmission destination (the terminal station 7), and a transmission source (the terminal station 6), as well as transmission request information. If the allocation of a frequency band is unavailable (No at Step S4), the feeder link station 4 repeatedly executes the processing at Steps S3 and S4.
After the transmission request signal from the feeder link station 4 is received, if approving the transmission request, the terminal station 7 then transmits a communication approval signal to the feeder link station 4 via the satellite repeater 5 (Step S6). Simultaneously, the terminal station 7 waits a signal from the terminal station 6 with the frequency band instructed in the transmission request signal from the feeder link station 4 (Step S6).
After receiving the communication approval signal from the terminal station 7, the feeder link station 4 transmits relay control information for performing relay control to the satellite repeater 5 such that the satellite repeater 5 can transmit the signal from the terminal station 6 in the service area 1 to the terminal station 7 in the service area 3 (Step S7).
After that, the feeder link station 4 transmits a communication permission signal to the terminal station 6 via the satellite repeater 5 (Step S8). The communication permission signal also includes frequency band information to be used by the terminal station 6 for communications.
The terminal station 6 then starts a communication to the terminal station 7 by using a frequency band instructed in the communication permission signal when receiving the communication permission signal from the feeder link station 4 (Step S9).
To carry out communications for control between the feeder link station 4 and the terminal stations 6 and 7 in the above sequence, fixed channels allocated for the control is used.
It is desirable in recent satellite communications that signals in various bandwidths, such as an audio signal and an image signal, are transmitted and received by efficiently using frequency resources between terminals (and also between the terminals and the feeder link station in a case of a double-hop system). Therefore, it is expected that a high efficiency in communications is achieved and a communication capacity of a system is increased within limited frequency resources flexibly coping with variations in traffic from a low-speed audio signal to a high-speed data communication.
As a conventional technology that achieves an efficient use of the frequency resources, there is a technology described in a non-patent document 1 described below. For example, according to a satellite system using a through repeater satellite, a frequency bandwidth BWd of a down-link beam in a service area is as follows:BWd=[bandwidth of each up-link beam BWu]×[number of service areas]However, according to the non-patent document 1, the same communication volume as a conventional one is achieved by compressing the frequency bandwidth of the down-link beam through a cluster multiplexing.
FIG. 9 is a schematic diagram that depicts an operation example of a system according to the non-patent document 1. The left graph of the two graphs depicts an operation example of an up-link in each service area, and also depicts an operation example of a down-link when not using a method according to the non-patent document 1. On the other hand, the right graph depicts an operation example of a down-link when using the method according to the non-patent document 1. A vertical axis in each of the graphs indicates the frequency of a down-link beam to the service area 1, and a horizontal axis indicates time.
According to FIG. 9, rectangles A to G indicate all data transmitted to the service area 1 from the service areas 1 to 3 with time and frequency directions. The vertical axis of each of the rectangles indicates a frequency band to be used for transmission of data. The data A and B are transmission data from the service area 1 to the service area 1, the data C and D are transmission data from the service area 2 to the service area 1, and the data E and F are transmission data from the service area 3 to the service area 1. Upward arrows “⇑” shown under the time axis in each of the graphs indicate time points at each of which a transmission request for each data is made.
According to a left graph in FIG. 9, the frequency bandwidth BWd required for the down-link to the service area 1 is “BWu×3”. On the other hand, according to the method of the non-patent document, frequencies of signals from respective up-links are switched by the satellite repeater 5, the frequencies are rearranged (compressed in the frequency axis direction) such that the frequencies are packed to eliminate unused intervals, and then the signals are transmitted to the service area 1. A right graph in FIG. 9 depicts a state of each data on the down-link when the frequencies are compressed.
The satellite repeater 5 extracts only necessary data from signals from up-links, and packs the signals in the frequency axis direction, so that the down-link frequency bandwidth allocated to the service area 1 from each of the service areas is compressed from BWu to BWc (BWu>BWc). As clearly shown in FIG. 9, the down-link frequency bandwidth to the service area 1 is reduced to BWd′(=BWc×3) from BWd without any trouble in communications because of the rearrangement of frequencies performed by the satellite repeater 5.
In this way, according to the non-patent document 1, the efficiency of use of frequencies is improved by compressing a down-link frequency bandwidth from a satellite when the satellite repeater 5 switches frequencies.
Non-Patent Document 1: “Equipment Technology in Next Generation Mobile Satellite Communication System”, Technical Study Report, SAT2003-113, Institute of Electronics, Information and Communication Engineers